1. [Technical Field of the Invention]
The present invention relates to an image pickup apparatus. More specifically, the present invention relates to an image pickup apparatus which can be used for a video camera having a motion-picture mode and a still-picture mode.
2. [Prior Art]
Video cameras for taking motion pictures use solid-state image pickup elements such as CCD image sensors whose pixel array chiefly complies with the NTSC/PAL for output signal formats. For example, a video camera based on the DV format, one of home-use digital VTR standards, generally uses approximately 340,000 to 420,000 pixels.
Image pickup using a solid-state image pickup element such as a CCD image sensor is equivalent to spatial sampling. It is a common practice to install an optical low-pass filter in an image pickup optical system for preventing image quality degradation due to aliasing components such as jagged edges in an image and false colors in a high-frequency portion. These aliasing components result from a sampling carrier caused by a pixel pitch itself or caused by repeated color filters formed on pixels.
The above-mentioned optical low-pass filter cannot easily provide flat frequency characteristics up to a high frequency and decreases high frequency components in an image, degrading the resolution.
Especially, a color camera system using a single CCD allows a single CCD image sensor to generate the brightness and colors, forming different color filters for respective pixels. It is necessary to prevent image quality degradation due to aliasing components resulting from a sampling carrier generated by this color coding. For this purpose, such a color camera system must limit the band to a low spatial frequency, further degrading the resolution, in contrast to a camera system without having the color coding. For example, the color coding of two repetitions such as xe2x80x9cYe, Cy, Ye, Cyxe2x80x9d must limit the band to a half of the frequency for a monochrome image pickup system or a 3-CCD camera system which separates light into RGB components using a prism.
The NTSC/PAL for output signal formats specifies the number of pixels in an array. Some CCD image sensors follow this specification. Most of these CCD image sensors use rectangular pixels. This means that an NTSC color system, for example, often uses 720 pixels (horizontal) by 480 lines (vertical) as an effective combination of the horizontal resolution (the number of samples) and the number of lines in a vertical direction within an effective display screen. The NTSC color system displays these pixels on a standard television screen having a horizontal-vertical ratio of 4:3. This provides a single pixel in the CCD image sensor with a tall shape having a horizontal-vertical ratio of 8:9. If this single pixel is handled as is and is displayed on a PC (personal computer) monitor which processes pixels as tetragonal lattices, an image is displayed with an incorrect aspect ratio (vertical-horizontal ration). A pixel number conversion is required for display with a correct aspect ratio.
A CCD image sensor for video cameras generally uses the different numbers of pixels in a vertical direction depending on scanning densities for various NTSC or PAL format. This makes cost-saving difficult.
When an image picked up by the CD image sensor is electronically enlarged, i.e. electronically zoomed, the image is degraded the resolution in inverse proportion to the magnification. This causes a disadvantage of providing a poor image quality compared to zooming through the use of an optical lens.
The present invention has been made in consideration of the foregoing. It is an object of the present invention to provide an image pickup apparatus which can: increase resolutions by improving output image characteristics at high frequencies; easily generate an image with pixels as tetragonal lattices; use a CCD image sensor for both NTSC and PAL video format; and perform electronic zooming without image quality degradation.
For solving the above-mentioned problems, an image pickup apparatus according to the present invention is characterized by having: an image pickup element which uses more pixels than those in an output image; and pixel number conversion means which converts an intermediate image signal to an output image signal using a pixel number reduction conversion.
Therein, the intermediate image signal which is created from an image pickup signal of the above-mentioned image pickup element contains more pixels than those in the output image.
The above-mentioned image pickup apparatus has a motion-picture mode and a still-picture mode. In the motion-picture mode, the image pickup apparatus extracts a signal for the intermediate image from an image pickup area in the image pickup element. The intermediate image contains fewer pixel than contained in the image pickup element and more pixels than contained in the output image. In the still-picture mode, the image pickup apparatus reads the entire image pickup area in the image pickup element to create a signal for the intermediate image.
The above-mentioned image pickup element uses a CCD image sensor which provides tetragonal-lattice pixels.
The above-mentioned pixel number conversion means performs zooming by varying conversion ratios in the intermediate image signal.
The image pickup apparatus according to the present invention is provided with an image pickup element which uses more pixels than those in an output image; and pixel number conversion means which converts an intermediate image signal to an output image signal using a pixel number reduction conversion.
Therein, the intermediate image signal which is created from an image pickup signal of the above-mentioned image pickup element contains more pixels than those in the output image.
Therefore, the image pickup apparatus can improve image characteristics at high frequencies and increase resolution or sharpness.
The image pickup apparatus has a motion-picture mode and a still-picture mode. In the motion-picture mode, the image pickup apparatus extracts a signal for the intermediate image from an image pickup area in the image pickup element. The intermediate image contains fewer pixels than contained in the image pickup element and more pixels than contained in the output image. In the still-picture mode, the image pickup apparatus reads the entire image pickup area in the image pickup element to create a signal for the intermediate image. Consequently, in the motion-picture mode, the image pickup apparatus can perform a camera-shake correction and generate an output image with improved resolution or sharpness of the read image. In the still-picture mode, the image pickup apparatus can provide an output image by optimizing the image pickup element resolution.
The above-mentioned image pickup element uses a CCD image sensor which provides tetragonal-lattice pixels. Using this image pickup element, the image pickup apparatus can easily display images with a correct aspect ratio on a PC (personal computer) monitor. Different output image formats are used for NTSC/PAL systems. By varying ratios of the above-mentioned pixel number conversion, it is possible to use a CCD image sensor common to both systems, easily switch between these systems, and provide advantageous cost saving.
In addition, zooming is available by varying conversion ratios of the above-mentioned pixel number conversion means. A digital camera using a single-focus lens can perform zooming with minimal image quality degradation. A digital camera using an optical zoom lens can increase zoom magnifications.